The specific aims of this proposed research project are to test the effets of unilateral trigeminal motor nucleus (TMNu) lesions upon (1) mandibular movements during mastication, (2) muscle activity (EMG) patterns during mastication, and (3) craniofacial form (muscle-bone alterations). Unilateral electrolytic lesions of the TMNu will be produced in actively growing laboratory rats (Sprague-Dawley strain) at 40 days of age. After sacrifice, muscles of mastication will be dissected and weighed to the nearest 0.1 g in order to document the extent of postlesion muscle atrophy. Both experimental and control rats will provide dry skull preparations for osteometric analyses and brain histology preparations for documentation of lesion extent. Before sacrifice, rats in each experimental group will provide masticatory EMG and mandibular movement data preoperative and at 4, 12, and 20 days postlesion; some rats will also provide these data at 35, 55, 63, and 83 days postlesion. Frontal and sagittal plane mandibular movement data will be obtained by the optoelectronic method and simultaneously recorded with EMG data from left and right side muscles of mastication. After digitizing and computer analyses, these physiologic data will be correlated with postoperative morphological data to assist interpretations of the lesion effects. Data from this proposed research project would be of value to clinicians and basic researchers in dentistry, orthodontics, maxillofacial surgery, neurophysiology, and functional anatomy. Such data will provide important new detailed information on the actual role of the CNS and muscle function in the development of craniofacial form. Since hemifacial microsomia, human skull asymmetry, and associated TMJ dysfunctions are strongly correlated with atrophy of muscles of mastication, detailed neuromuscular physiologic data correlated with morphological changes will prove useful in the establishment of future treatments for human craniofacial growth and masticatory dysfunctions.